Internal combustion engines are typically started or “cranked” through the use of an electric starter system. The electric starter system typically includes an electric motor having a rotational output such as a shaft, a source of electrical energy for the electric motor such as a DC battery, and a mechanical interface such as a gear train between the electric motor and the engine suitable for transferring the rotational output of the electric motor to the engine. The rotational energy produced by the electric motor is transferred with an amount of torque and angular velocity sufficient to turn a primary shaft of the motor such as its crankshaft and initiate internal combustion, after which time the operation of the engine is sustained by dedicated fuel-air and spark delivery systems.
Starter systems including electric starter motors and gear trains are presently being implemented with powered equipment such as lawnmowers, whose small engines have traditionally been manually started by means of a pull cord. For powered equipment such as lawnmowers and the like, conventional starter systems have included two-stage gear arrangements.
Referring now to FIG. 1, a schematic illustration is given for a conventional two-stage gear arrangement, generally designated 10, that typically is employed for starter assemblies. Two-stage gear arrangement 10 consists of a first gear G1 which can be intermeshed with a second gear G2 that in turn can be intermeshed with a third gear G3. Although not specifically shown, it will be appreciated by persons skilled in the art that each gear G1–G3 has a series of teeth formed around its periphery, such that the teeth of each gear G1–G3 engage the teeth of an adjacent gear G1–G3. Accordingly, as a result of these intermeshing engagements, rotation of first gear G1 causes rotation of second gear G2, and rotation of second gear G2 likewise causes rotation of third gear G3. Alternatively, gears G1–G3 could be non-toothed wheels constructed in whole or in part from a suitable frictional material such as rubber.
First gear G1 represents a pinion gear that is typically coaxially attached to an output or starter motor shaft SMS of a starter motor (not shown). Second gear G2 is typically an idler gear attached to a rotatable stub shaft SS. Third gear G3 represents a large-diameter gear that is typically coaxially attached to an engine shaft ES of an internal combustion engine (not shown), such as the engine of a lawnmower. Engine shaft ES is typically a crankshaft that is directly coupled to the engine. In the case of a lawnmower, one or more cutting blades can be directly attached to the crankshaft such that the crankshaft provides the torque needed to drive the cutting blades. Alternatively, engine shaft ES is an output shaft that is coupled to the crankshaft through a clutch or other suitable means for selectively engaging and disengaging the output power of the engine from the cutting blades to control the operation of the cutting blades. First gear G1, second gear G2, third gear G3, and their associated shafts SMS, SS, and ES rotate about separate axes of rotation A1, A2, and A3, respectively.
The starter motor is typically an electric DC motor that is energized either through connection with a battery or to a suitable electrical power source such as an electrical outlet providing standard AC line voltage. In the case where the electric motor is to be connected to an electrical outlet, an AC/DC converter is required on the input side of the electric motor to provide rectified DC power to the electric motor. Alternatively, the starter motor could be an AC motor.
In operation, the starter motor is energized to rotate first gear G1, thereby forcing second gear G2 and third gear G3 to rotate. Because third gear G3 is fixedly attached to engine shaft ES, third gear G3 drives the rotation of engine shaft ES. As is appreciated by persons skilled in the art, the rotation of engine shaft ES causes one or more internally disposed pistons communicating with engine shaft ES to reciprocate. The resulting reciprocation of the piston, in combination with the priming of the engine with an appropriate fuel-air mixture and the discharging of a spark plug communicating with the cylinder enclosing the piston, initiates combustion within the cylinder and thereby starts the engine. Typically, a suitable means is provided for decoupling third gear G3 or second gear G2 from the remaining portion of the gear train, so that starter motor shaft SMS of the starter motor ceases to rotate once the engine has been started.
Typically, the resultant gear ratio (also termed the angular-velocity ratio) provided by two-stage gear arrangement 10 is 19.4:1, which is the ratio of the input angular velocity (that of first gear G1) to the output angular velocity (that of third gear G3). As is appreciated by persons skilled in the art, the gear ratio of any gear train depends on the number of teeth provided on each participating gear. Two-stage gear arrangements such as gear arrangement 10 illustrated in FIG. 1 typically require the use of a high-torque, low-rpm (300–600 rpm) electric motor to successfully turn engine shaft ES and consequently start the engine.
A mechanism for decoupling the starter motor from its associated engine can be provided to prevent the output shaft of a starter motor from needlessly rotating after its associated engine has been started and possibly damaging or wearing out the starter motor. Examples of starter assemblies that include two-stage gear arrangements and decoupling mechanisms are disclosed in U.S. Pat. No. 4,507,566 to Leatherman et al. and U.S. Pat. No. 5,755,137 to Tomida. Leatherman et al. disclose a starter assembly for a gasoline engine-powered lawnmower in which a starter assembly is pivotably mounted adjacent to the engine and above a mower deck that houses cutting blades. The starter assembly includes an electric starter motor, an output shaft, and an associated pinion gear, which together are movable along an arcuate slot between engaged and disengaged positions. An operator manipulates a bell crank and associated linkage to move the starter assembly into the engaged position, in which the pinion gear meshes with a gear rotatably connected with the engine. Tomida discloses a starter assembly typically used in larger engines provided with motor vehicles. The starter assembly of Tomida utilizes a magnet switch to actuate the pinion gear of the starter assembly into engagement with a ring gear rotatably connected with the engine through its crankshaft.
As in the case of a typical two-stage gear arrangement such as two-stage gear arrangement 10, the two-stage gear assemblies of powered equipment such as disclosed by Leatherman et al. and Tomida typically require the use of a high-torque, low-rpm electric motor. In some cases, however, the use of a high-torque, low-rpm electric motor as the starter motor is not desirable. In particular, it is proposed herein to employ a high-performance battery such as a nickel-cadmium (NiCd or “NiCad”), nickel metal hydride, or lithium ion battery in conjunction with engine-powered equipment to provide stored energy for an electric starter apparatus. It has been found that the use of a NiCd battery in conjunction with a high-torque, low-rpm electric motor does not provide acceptable reliability and performance. Accordingly, it is proposed to provide a lower-torque/higher-rpm electric starter motor for use in conjunction with the NiCd or other high-performance battery. However, it has further been found that a conventional two-stage gear arrangement such as illustrated in FIG. 1 and disclosed by Leatherman et al. and Tomida is not compatible with lower-torque/higher-rpm electric starter motors. Therefore, an acknowledged need exists in the art for an improved gear arrangement that can support the use of a low-torque/high-rpm electric starter motor.